The relative rates of formation of 4-hydroxy propranolol (40H Pr), 5-hydroxy propranolol (50H Pr) and desisopropyl propranolol (DI Pr), from the R-isomer of propranolol, a widely used Beta-adrenergic blocker, varies with the treatment with the animal species. A monoclonal antibody raised against cytocharome P-450 induced by 3-methyl cholanthrene inhibited the metabolism of the R isomer of propranolol by liver microsomes from rats, mice, guinea pigs and rabbits treated with Beta-naphthoflavone (BetaNF) but it did not inhibit propranolol metabolism by liver microsomes from hamsters, treated with BetaNF. Since the relative amounts of 40H Pr, 50H Pr and DI Pr inhibited by the antibody differed with the animal species, it seems likely that the antibody inhibits different forms of cytochrome P-450.